Propulsion, materials, aerodynamics, and thermal management: the techniques for achieving hypersonic speeds and developing hypersonic aircraft.
Hypersonic speed, beyond Mach 5, imposes aerodynamic, thermal, and structural constraints that are radically different from those encountered in supersonic flight. The development of hypersonic aircraft is both a strategic and scientific goal for many nations. It is no longer just a matter of designing a vehicle capable of briefly reaching these speeds, but of maintaining stable, controlled and reusable flight. To achieve this, engineers must master innovative propulsion solutions, new ultra-high-strength materials, integrated thermal management, and suitable aerodynamic architectures. This article provides a technical and detailed analysis of the main approaches used to make hypersonic flight possible, as well as the programs currently underway and the obstacles that remain to be overcome.
Rocket engine propulsion
The rocket engine remains the most direct system for achieving hypersonic speeds, as it produces thrust independently of the ambient air by carrying its own oxidizer. It operates on the principle of burning a liquid or solid fuel combined with an onboard oxidizer. This autonomy allows it to deliver massive thrust, but at the cost of extremely high fuel consumption and reduced range. For a hypersonic aircraft, this mode of propulsion has obvious limitations: carrying large quantities of propellants adds weight to the airframe and reduces operational capabilities. The rocket engine is most effective during rapid ascent and atmospheric entry phases, or for hypersonic missiles designed for rapid strikes. However, it remains unsuitable for prolonged atmospheric flight, as the weight carried cannot be offset by reasonable fuel tanks. The future of rocket engines in hypersonic flight therefore lies in their integration into hybrid systems, where they serve as initial accelerators before giving way to ramjet or scramjet propulsion.
Ramjets and their limitations
The ramjet is based on a different principle: it does not carry any oxidizer, but uses air compressed by the speed of the aircraft to enable fuel combustion. Its simple design, with no compressors or turbines, makes it a lightweight and robust solution. It becomes truly effective at speeds of Mach 3 and can reach Mach 5. However, as combustion takes place at subsonic speeds, this engine loses stability beyond this limit, as the sudden deceleration of the air causes thermodynamic instabilities. In the context of a hypersonic aircraft, this technology is therefore only an intermediate step. Static jet engines are nevertheless used in high-speed cruise missiles and offer attractive energy density for medium-range missions. Their role in future programs is limited to ensuring a transition to higher speeds by providing the necessary thrust before switching to scramjet. Their mechanical simplicity remains an advantage, but their inability to maintain stable combustion beyond Mach 5 excludes them from the field of sustainable hypersonic flight.
The scramjet, the heart of hypersonic flight
The supersonic combustion ramjet, or scramjet, is the real key to hypersonic aircraft. Unlike the ramjet, it maintains a supersonic airflow in the combustion chamber. This allows it to operate efficiently at Mach 7, 8 or even 10, using atmospheric oxygen as an oxidizer. The fuel, usually hydrogen or a light hydrocarbon, must ignite and burn in a matter of milliseconds, which is a major challenge in terms of fluid dynamics and combustion chemistry. The scramjet offers much higher mass efficiency than the rocket engine, as it does not require the carriage of oxidizer, but it remains extremely sensitive to flow quality and turbulence phenomena. Demonstrators such as the X-51 have validated the possibility of maintaining stable scramjet flight for several minutes, but the transition between different speed regimes remains a critical issue. The scramjet is currently the focus of all research, as it is the only system capable of sustaining continuous atmospheric flight at hypersonic speeds.
Combined cycles to expand the flight envelope
A hypersonic aircraft must be able to take off, accelerate gradually, and reach Mach 10 in a controlled manner. No single engine can cover this entire operating range. The solution being considered is combined cycles. The TBCC (Turbine Based Combined Cycle) concept combines a conventional turbojet engine for takeoff and subsonic speeds, a ramjet for the transonic and supersonic zones, and a scramjet for the hypersonic phase. This system ensures a continuous transition and avoids thrust interruptions. The British SABRE concept explores another approach, with a hybrid engine capable of operating both in the atmosphere and in exoatmospheric rocket mode, thanks to an ultra-fast pre-cooler that lowers the air temperature before it enters the engine. However, combined cycles pose enormous technical challenges, as each system must coexist in a compact structure without causing performance losses. This complexity is currently one of the main obstacles to manned hypersonic flight, but it is the most promising route for intercontinental transport at hypersonic speeds.
Aerodynamics specific to hypersonic flight
At hypersonic speeds, aerodynamic phenomena change scale. Shock waves dominate the flow, reducing lift and significantly increasing drag. Engineers favor waverider architectures, whose geometry exploits the shock wave generated by the nose of the aircraft to produce additional lift. These shapes are characterized by tapered, very elongated, and flattened fuselages. Conventional wings lose their effectiveness and become secondary to the load-bearing fuselage. Traditional control surfaces, which are not very effective in such a flow, must be supplemented by thrust vectoring systems, which allow the trajectory to be controlled by directly orienting the engine jets. Hypersonic aerodynamics also require constant optimization between stability, energy efficiency, and thermal management. Every curve in the fuselage influences pressure distribution and therefore the location of hot spots. Designing a hypersonic aircraft requires thousands of hours of computational fluid dynamics and specialized wind tunnel testing capable of reproducing Mach 7 to 10 speeds.
Materials and their thermal resistance
Hypersonic speed causes intense heating due to air friction, reaching 2,000 to 3,000°C in certain areas. The materials must therefore be able to withstand extreme thermomechanical stresses. Aluminum alloys are unsuitable and are being replaced by titanium, nickel-based alloys such as Inconel, and carbon-carbon composites. The latter have already been used on the tiles of the space shuttle. Researchers are also studying ultra-refractory ceramics such as zirconium carbide and hafnium carbide, which can withstand temperatures of over 3,000°C. In addition to pure resistance, mechanical performance under thermal cycling is crucial: materials must tolerate differential expansion without cracking. To reinforce this resistance, some systems use regenerative cooling, where fuel circulates in the walls to absorb heat before being injected into the engine. This dual function improves both energy efficiency and thermal performance. Without these innovations, no hypersonic aircraft could perform multiple consecutive missions without structural degradation.
Thermal management systems
Thermal management is a key issue in hypersonic flight. Temperature gradients between the nose, leading edges and the rest of the airframe can reach several hundred degrees. Engineers use ablative heat shields, inspired by space capsules, which gradually burn away as they dissipate energy. However, this solution limits reuse and remains unsuitable for a hypersonic aircraft. Other approaches favor fuel circulation in exchangers integrated into the walls, which allows a large part of the heat to be absorbed before injection into the combustion chamber. This process, called active cooling, optimizes consumption and protects the structure. Thermal management also involves extensive integration between mechanical, electrical, and electronic systems. Sensors, computers, and flight controls must function despite extreme ambient temperatures. To achieve this, circuits are being developed that are protected by insulating casings or actively cooled. Thermal management is therefore a comprehensive system, without which hypersonic aircraft could not ensure stable flight or guarantee operational safety.
Current programs around the world
The development of hypersonic speed has become a major geopolitical issue. The United States has conducted the X-43 and X-51 programs and is currently developing the ARRW, while also working on the SR-72 hypersonic aircraft concept. China has tested the DF-ZF glider and deployed the DF-17 missile, capable of atmospheric maneuvers at Mach 10. Russia is promoting the Avangard, an intercontinental hypersonic glider, and the Zircon missile. Europe, through the ESA and industrial partners, is working on HEXAFLY-INT to validate the basics of civil hypersonic transport. India is developing its HSTDV, a demonstrator equipped with a scramjet, which already flew in 2020. These programs illustrate a global competition in which hypersonics are seen as a strategic breakthrough. Hypersonic missiles are already a military reality, but manned hypersonic aircraft remain in the experimental phase. Demonstrators are accumulating data to prepare for the transition to operational aircraft, but it will take several more decades to overcome the current limitations and enter an industrial phase.
Remaining prospects and challenges
The hypersonic aircraft remains a project for the future. The main challenges concern the stability of scramjets, the durability of materials subjected to repeated cycles, and the reliability of combined cycle architectures. Cost management is also a major obstacle: such an aircraft requires specific infrastructure, suitable fuel, and complex logistics. In the civilian sector, the idea of a two-hour flight from Paris to Tokyo is appealing, but it raises questions about safety, noise, energy consumption, and environmental impact. In the military sector, hypersonic missiles are already operational, but transferring these capabilities to a reusable aircraft requires a further technological leap. Researchers estimate that a manned hypersonic aircraft demonstrator could be ready by 2040 if progress in propulsion and materials continues at the current pace. The future of hypersonic flight therefore lies in the convergence of these technologies and in the ability of governments and industry to finance and secure this type of program in the long term.
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